Abstract Tyrosine was shown to be an essential amino acid for protein synthesis in primary chick fibroblast tissue cultures. When C14- l -tyrosine was added to the tissue cultures at the time of high-multiplicity infection with Sindbis virus, the specific activities of tyrosine in the hydrolyzate of viral protein and in the medium were similar. Cells prelabeled with C14-tyrosine and then infected in unlabeled medium made only a minimal contribution of radioactivity to the viral protein. Similar results were obtained with C14- l -isoleucine. Most of the protein of Sindbis virus thus appears to be synthesized from amino acids present in the medium at the time of infection, although the viral phospholipid is derived from cellular material.